A 6 MGD reverse osmosis plant in South Texas that was using chlorine to oxidize arsenic for improved membrane rejection was experiencing severe membrane fouling. The ferrous ions in the well water were being oxidized by the chlorine, resulting in ferric hydroxide fouling of the cartridge filters and membranes throughout the system. The RO facility did not have an acid dosing system and the antiscalant in use was not designed for such high ferric concentrations.